In the operation of an internal combustion engine, there are four steps that must take place: fuel and air must be introduced into a cylinder, the mixture must be compressed, it must then be burnt, and the exhaust gasses must be removed from the cylinder before introducing a fresh charge of fuel and air. In diesel engines, the fuel and air do not enter the cylinder at the same time, but the cycle of operation is nevertheless the same. There are two basic systems of accomplishing these operations, the four stroke cycle, in which one operation takes place during each passage of the piston up or down the cylinder, and the two stroke cycle in which two of the operations are accomplished in each passage of the piston.
In a four stroke cycle, the four mentioned steps require two complete reciprocations of the piston or two revolutions of an associated crank shaft. A fly wheel stores sufficient energy from a power stroke to carry the piston through the next three strokes before the next power stroke. In a two stroke cycle, a first charge of fuel and air is compressed below the piston or by some other means and then forced into the cylinder when the piston is at the bottom of its stroke; the charge is then compressed by the pistons upward motion, and ignited at the end of its compression stroke. At the end of the power stroke, a fresh charge sweeps the exhaust gasses out of the cylinder. However some of this charge is lost through the exhaust port during this sweeping. Accordingly, there is one power stroke for each reciprocation of the piston or revolution of an associated crank shaft.
The advantages of a two stroke engine over a four stroke engine are that it provides more frequent power strokes and has greater mechanical simplicity and lightness. These advantages are to some extent offset by the fact that the two stroke engine wastes a large portion of the charge of fuel and air admitted into the cylinder. If the charge is of the same size as would be required by a comparable four stroke engine, the two stroke engine would not sweep out exhaust gasses completely, thereby cutting down on the power developed on the next stroke since a percentage of the charge includes burnt gases from the previous cycle. In addition the power stroke is shorter, since the exhaust gasses are expelled during part of the down stroke. A further disadvantage of the two stroke engine is that of lubrication. In a four stroke engine, oil stored in a crank case splashes up onto the cylinder walls to lubricate the piston. However such a system cannot be used in two stroke engines as oil splashed onto the cylinder would be carried out with the exhaust gasses eventually leaving the piston unlubricated. This is overcome by mixing lubricating oil with the fuel. However, this leads to smoky exhaust fumes and fouling of the engine by partially burnt oil.
For the above reasons, the two stroke cycle is preferred for small engines where lightness and simplicity are more important than the problems of highly polluted exhaust and the necessity of mixing lubricating oil with fuel, for example engines for lawn mowers, motor cycles, and tools such as chain saws and brush cutters. The four. Stroke cycle is favoured for higher powered engines, for example, in motor vehicles and boats where several pistons are attached to a crank shaft providing more power strokes per turn.